Question

Which phase transition requires more heat, melting or freezing, if the mass of water/ice in both cases is `"35.0 g"`?

Answer 1

Neither.

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Recall that the freezing point IS the melting point. That means at `0^@ "C"`, the melting/freezing point of water, we really have:

`q = nDeltaH_"trs"`,

where:

• `q` is the heat absorbed to melt the solid, or heat released due to freezing the water.
• `DeltaH_"trs"` is the enthalpy for the phase transition.
• `n` is the `"mol"`s of water in this process.

So, if the masses are both `"35.0 g"`, then the `"mol"`s of water are equal in either case:

`q_"frz" = n_1DeltaH_"frz"`

`q_"mlt" = n_2DeltaH_"mlt"`

where `n_1 = n_2 = n`.

But since `DeltaH_"mlt" = -DeltaH_"frz" = "6.02 kJ/mol"`, we really have that `|q_"frz"| = |q_"mlt"|`. Therefore, both involve the same amount of heat.

Which `q` is positive, `q_"frz"` or `q_"mlt"`?